1. Field of the Invention
The subject invention relates to transducers, and to methods of making transducers, having a Wheatstone bridge, to methods of reducing thermal shift in Wheatstone bridge-type transducers, to pressure transducers, to methods of making pressure transducers, to methods of reducing thermal shift in pressure transducers, to pressure and other transducers having reduced thermal shift, to disposable pressure and other transducers, and to physiological pressure transducers.
2. Information Disclosure Statement
The following disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also, the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments which may be subsequent in time or priority.
Transducers of the type herein considered are devices that convert a physical variable, such as pressure, into a corresponding electric signal. Frequently, strain gages are employed for that purpose on a flexible diaphragm or beam stressed or strained by the physical variable. The strain gages or strain gage resistors are most typically interconnected in a Wheatstone bridge arrangement, but half bridges and other arrangements are also common.
Two well-known factors that affect strain gage performance are thermal zero shift (TZS) and thermal sensitivity shift (TSS), which require compensation in one form or another, usually through trimming or addition of shunt and series resistors to different legs of the bridge or strain gage assembly.
In practice, the requisite balancing and compensation adds at least one further step to the manufacture of transducers of the type here under consideration and sometimes requires further adjustment in the field or in subsequent calibration. In the case of large and expensive transducers, that extra work and procedure arguably could be viewed as a worthwhile investment in optimized performance. However, in the case of disposable transducers, for instance, such an argument becomes tenuous, as time and equipment investment is increasingly hard to justify for a disposable transducer that is used only once.
This, in turn, generates the danger that a transducer which was built as a disposable item will be used more than once. In practice, this easily leads to faulty measurements and worse consequences.
For instance, in the health-care field, there is an increasing concern with rising costs. The temptation is great, therefore, for health-care personnel to use a disposable blood pressure or other transducer repeatedly, if the expense of that transducer militates against disposal thereof after only one use. Since, however, such disposable transducers, in order to keep costs low, are not designed for resterilization and reuse, there is not only a danger of faulty measurements and misleading data, but also of harm to patients through continued use of resterilized equipment that was not designed to be resterilized.
Some relief in the direction of a solution appeared with the development of semiconductor transducer diaphragms or beam, in which strain gages were diffused through appropriate doping of a semiconductor wafer. Since a large number of such diaphragms or beam were thus manufactured on a single wafer and were then dissected therefrom, the individual diaphragms or beams could be made small enough so as to display a fairly uniform temperature coefficient of resistance therethrough. However, existing designs reintroduced the need for thermal shift compensation through imposition of mechanical strains on the semiconductor diaphragm or beam.
Despite mounting of diaphragms with the aid of spaced steel balls and despite a known proposal to mount a Hall effect transducer on a rubber support, no conception of the subject matter hereinafter claimed did, however, become apparent. It is thus a curious fact that efforts to calibrate even disposable semiconductive transducers in the traditional way, or efforts to provide expensive and complex equipment for that purpose, continued prior to, and even after conception of, the subject invention. That, at any rate, would have nullified whatever benefit a mechanically isolated or floating condition would have had on thermal shift.